This invention relates to the processing of semicondcutor materials and, more particularly, to a process for reactive ion etching.
III-V semiconductors and their derivative ternary and quaternary compounds are of interest for the fabrication of both high speed switching devices and optoelectronic elements such as lasers and detectors. The necessity for efficient light coupling, confinement and/or transmission puts stringent requirements on the resolution of the pattern transfer technique and on the edge smoothness of the structures formed. Integration of these devices into compact circuits combining high speed electrical and optical processing requires the development of a fabrication technology comparable to that used for silicon integrated circuits.
Preferential chemical etching has been widely used to provide the requisite smooth walls, but patterning III-V compounds in this way requires the precise orientation of the crystalline substrates and thus limits device design possibilities. Gratings selectively etched into GaAs have shown slight sidewall modulation attributed in part to misalignment between the grating direction as defined by the mask, and the cleavage planes, see, for example, an article entitled "Selectively Etched Diffraction Gratings in GaAs", Applied Physics Letters, Vol. 25, No. 4, Aug. 15, 1974, pp. 208-210 by K. Comerford and P. Tory. Variation in the depths of preferentially etched channel waveguides according to the initial stripe width in the defining mask has been reported in an article entitled "Optical Waveguides Fabricated by Preferential Etching", Applied Optics, Vol. 14, No. 5, May 1975, pp. 1200-1206 by W. T. Tsang, C. C. Tseng, and S. Wang.
Optical gratings have also been produced by ion-beam milling substrates through photoresist masks. Limitations of this technique are possible facetting of the substrate, redeposition of sputtered material (hence alteration of etched profiles) and enhancement of lattice defects, see Tsang et al referred to hereinabove and an article entitled "Profile Control by Reactive Ion Sputter Etching" J. Vac. Sci. Technol., 15 (2), March/April 1978, pp. 319-326 by H. W. Lehmann and R. Widmer.
Reactive ion etching has been used to transfer high resolution sub-micron patterns into Si, SiO.sub.2, and Si.sub.3 N.sub.4 substrates. One example of this is the patterning of square wave gratings 250 A deep with 3200 A periodicity into SiO.sub.2 see, for example, an article entitled "Alignment of Liquid Crystals Using Semiconductor Periodicity Gratings", Applied Physics Letters, Vol. 32, No. 10, May 15, 1978, pp. 597-598 by D. C. Flanders, D. C. Sharer and H. I. Smith. The utility of this technique for the fabrication of component devices for integrated optics is manifest, however, efforts to reactively ion etch III-V compounds have thus far met with limited success, see, for example, an article entitled "A Survey of Plasma-Etching Processes", Solid State Technology, 19 (5), May 1976, pp. 31-36, by R. L. Bersin.